Boating Life

How To: Use International Maritime Signal Flags

November 15, 2021
Ingman Marine
How To Guides
November 15, 2021
Ingman Marine
How To Guides

When at sea, the rules of communication change a bit, especially in international waters. Because of the possibility of language barriers and the potential need to convey messages quickly, having a way to communicate universally can become crucial. This is where international maritime signal flags become useful. These flags are part of the International Code of Signals (ICS) and are used by vessels to relay important information regarding safety of navigation and emergency situations.

We’ve put together this guide to help you recognize the signals. Let’s decipher them!

International Code of Signals

The International Code of Signals (ICS) is a collection of codes and signals created by the British Board of Trade to give captains and mariners a universal language. It is the latest iteration of a set of consolidated signaling and communication systems and is maintained by the International Maritime Organization. As part of the code, vessels should fly the code/answering pennant to let others know they are using ICS to communicate.

Intro to International Maritime Signal Flags

There are 26 square flags (each representing a letter of the alphabet), ten numeral pennants, a code/answering pennant and three substitute/repeater flags. The flags correlate to a standardized alphabet and particular messages that were developed as part of the ICS. The only colors used for the signals are blue, yellow, red, black and white because they are easily recognizable at sea.

Boats and ships will fly anywhere between 1 and 7 international maritime signal flags, also referred to as nautical flags. Each flag or combination of flags has a different meaning; a message can be spelled out using a string of letter flags, or one or more flags can be used to form a code word whose meaning can be looked up in a code book held by both parties.

There are, however, standard messages known to mariners, which can be categorized as such:

  • Single-flag signals indicate very urgent or common messages.
  • Two-flag signals are most often used to signal distress or maneuvering.
  • Three-flag signals represent punctuation, verbs, general code and decode messages, points of the compass, standard times or relative bearings.
  • Four-flags signals are used for geographical representation, names of ships, and some other specific messages.
  • Five-flag signals are used to communicate time and position.
  • Six-flag signals indicate the cardinal directions (N, S, E & W) in latitude and longitude.
  • Seven-flags are for longitude signals containing more than one hundred degrees.

Flags: What They Mean



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Standard 2 & 3 Letter Flag Signals

AC – “I am abandoning the vessel.”

AD – “I am abandoning the vessel because of a nuclear accident. The vessel is a possible source of radiation.”

AN – “I need a doctor.”

AN 1 – “I need a doctor; severe burns involved.”

AN 2 – “I need a doctor; radiation casualties involved.”

BR – “I need a helicopter.”

CD – “I need immediate assistance.”

DV – “I am drifting.”

DX – “I am sinking.”

EF – “SOS/Mayday signal has been canceled.”

EL – “Repeat distress position.”

EL 1 – “What is the position of the vessel in distress?”

FA – “What is the position?”

GM – “The vessel cannot be saved.”

GN – “You should remove persons.”

GN 1 – “I wish some persons removed. Skeleton crew will remain on board.”

GN 2 – “I will remove persons.”

GN 3 – “Can you remove persons?”

GW – “Man overboard.”

IT – “I am on fire.”

JA – “I require firefighting appliances.”

JA 4 – “I require material for foam fire extinguishers.”

JL – “I have a risk of going aground.”

MAA – “I request urgent medical advice.”

MAB – “I request you to make rendezvous in position indicated.”

MAC – “I request you to arrange hospital admission.”

MAD – “I am . . . (indicate number) hours from the nearest port.”

MS 1 – “My vessel is a dangerous source of radiation; you may approach from my starboard side.”

LO – “Not in the correct position.”

NC – “I am in distress; immediate assistance is required.”

PD – “Navigation lights not visible.”

PP – “Keep clear.”

QD – “Going ahead.”

QT – “Going astern.”

QQ – “Health clearance required.”

QU – “No anchoring.”

QX – “Anchoring permission.”

UM – “The harbor is closed for traffic.”

US 4 – “No activity until the weather moderates.”

UP – “Requesting urgent permission to enter the harbor.”

Other Uses for Nautical Flags

NATO and other navies use many of these flags to signify the same meanings as illustrated above, but they also use certain combinations or their own unique flags to convey certain classified and unclassified messages known only to privy individuals.

Nautical racing also employs the flags, but they often have different meanings, signifying certain instructions to racers. For example, the S flag is used to represent a shortened course and the P flag alerts competitors to an imminent start.

Lastly, it’s important to keep in mind that when a vessel enters a specific nation’s territorial waters, a nation’s courtesy flag should be flown. Make sure the flag is in a good shape and it is hoisted on the starboard spreader.

Final Words

Knowing what nautical flags mean is vital for your safety at sea. There might be times when communication is available only this way and knowing which flags to use and when will keep you and your boat safe. These international nautical signals will help you communicate in any circumstance while at sea.

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